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A fundamental issue addressed in cosmology is the future of the universe—whether the universe will expand forever or eventually collapse. The first case (eternal expansion) is known as an open universe, and the second case (eventual collapse) is known as a closed universe. A closed universe would require sufficiently high density to cause gravity to eventually stop the universe’s expansion and begin its contraction.

Such a collapse would require a deviation from Hubble's law, so observational cosmologists try to observe the distances between very distant galaxies and Earth using methods other than measurement of redshifts. The scientists can then compare these distance measurements with the galaxies’ redshifts to see if Hubble’s law holds or not. In the late 1990s astronomers compared the redshifts of supernovas in distant galaxies. Surprisingly, distant supernovas were slightly fainter than had been expected. This result was tentatively interpreted as an acceleration of the expansion of the universe.

Astronomers were so surprised by the suggestion that the universe might be accelerating its expansion that they attempted to find other explanations for the relative dimness of distant supernovas, such as absorption by dust. By a few years into the 21st century, however, these other conceivable explanations had been ruled out, and the accelerating universe concept became widely accepted. The search continues to discover more and more distant supernovas.


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